I am an engineer and only took a minimum amount of physics in college and we really didn't get to space time. I picked up a book in the bookstore the other day that showed space time being bent around mass giving what we think of as gravity. But it really didn't say why. So that's what I am asking is why does mass bend space time?

All mass pulls against other mass with a force that we call gravity. Mass also pulls energy the same way. Another way of looking at this attraction is to not have an attractive force, but curvature in space-time around the masses, and all things, mass or energy, travel in a straight line through this curvature.

So the earth's orbit around the sun is a straight line within the space-time curvature that the sun creates.

I don't know of any definite theories as to WHY mass does this, whether you see it as a force or a bending of space-time.

Physics actually doesn't concern itself too much with these "why" questions. As long as our physical theories allow us to predict the behaviour of physical systems with high accuracy, that's all we ask of them.

So, nobody really knows why mass causes spacetime to curve. Maybe God likes it that way. Or maybe it's just chance that the laws of nature are set up that way. Or maybe space doesn't really curve at all, but the curved description of spacetime just works well and gives us good answers to gravitational problems.

Seems to me that space-time is curved only in mathematics, or in our descriptions. It's a way to make models work. I mean, it seems more common sense to say that gravity draws light and such into curved paths, rather than space being curved and light travelling straight. But the models work for figuring out solutions, so that's what people say...


And yes, I am one of those who thinks that when people say "it's counter-intuitive", it's crap. We humans were developed in this physical universe. Intuition is the result of billions of years of us interacting with reality.

Adam, there is no reason to think it is more logical that the path of light is bended than that space-time is.
The force that causes it is gravity...and gravity is a force between masses, but neither space nor light has any mass, both possibilities are pretty hard to imagine.
But i think there is a reason it is common to say that space is bended, scientist probably have a reason for it, I don't know exactly.

The reason we don't know exactly how the bending works is logical: we don't know how gravity works, and WHY I am sitting in my chair and not flying around.

I think physics does concern about the WHY? questions.
The purpose of all science is to answer this stuff.
The only problem is that physics are so complicated that we have to make certain models first, to come to an answer, we'll have to wait.

Originally posted by KneD
But i think there is a reason it is common to say that space is bended, scientist probably have a reason for it, I don't know exactly.
At low speeds, you can say quite equally that space is curved (general relativity), or that there's a force between you and the earth (newtonian gravity). There is no difference at low speeds -- both models give the same results.

At high speeds, or high gravitational field strengths though, you can no longer think about gravity as a simple force -- because the newtonian model can't explain observed things like the precession of Mercury's orbit, or the deflection of starlight, or the gravitational redshift. When you deal with high speeds, the only way to view gravity (that we know of) is as a curvature of space itself.

- Warren

because the newtonian model can't explain observed things like the precession of Mercury's orbit, or the deflection of starlight, or the gravitational redshift

yes it can, only you don't know it :)

Try sticking light's mass into Newton's equation and see what force you get.

Question: if curvature is a description of gravity without using forces, how does it explain gravitation redshift, which is a loss of kinetic energy by leaving starlight? What is the light pulling against, if there is no force, and it is heading straight out of the well, not along the curve?

Originally posted by Jaxom
Try sticking light's mass into Newton's equation and see what force you get.

Question: if curvature is a description of gravity without using forces, how does it explain gravitation redshift, which is a loss of kinetic energy by leaving starlight? What is the light pulling against, if there is no force, and it is heading straight out of the well, not along the curve?
Think of the space-time near a massive object (say, a star) as a bowl, with the star at the center. The light has to climb out of the potential well, up the side of bowl, to escape. It has to use energy to climb out of the potential well.

- Warren

Originally posted by c'est moi
yes it can, only you don't know it :)
Oh, I don't? Right. :rolleyes:

- Warren

warren, Paul gerber already calculated the advance of the perihelion of Mercury in 1898. you can arrive to einstein's equation without any relativity involved

-> Die Räumliche und zeitliche Ausbreitung der Gravitation. Von Paul Gerber. Zeitschrift für Mathematik und Physik Vol.
43, Pages 93-104 (1898).

hope your deutsh is well ;)

I found an explanation in GR as to why the wavelength of emitted light gets longer (redshift), it's due to time being slowed near a strong gravitation field, so the cycle of the light increases. Newtonian reasoning doesn't work there at all, since light should neither get bent nor have to pull against the star's gravity.

So I withdraw that question, but ask this: if the redshift is due to time slowing, then there should never be a blueshift as light goes into a gravity well, correct? Light should redden whenever it is in heavy gravity, regardless of direction?

Originally posted by c'est moi
warren, Paul gerber already calculated the advance of the perihelion of Mercury in 1898. you can arrive to einstein's equation without any relativity involved
"What Gerber did was to simply assume a rather arbitrary extra
velocity dependence of the Newtonian potential---that is, not
just that the potential depended on the retarded position, but
that it depended in a rather peculiar manner on the velocity of
the source. There seems to have been no particular physical
justification for this dependence---von Laue concluded that Gerber
probably worked backward from the answer, and Pauli characterized
it as "completely unsuccessful from the theoretical point of view."
In any case, it was not surprising that someone came up with a
formula that worked. With the discovery that the electromagnetic
potential had a velocity dependence, it was natural to guess that
the same might be true for gravity, and a dozen or so different
functional forms were tried by various physicists (none with any
particular physical justification). Gerber was lucky enough to
come up with the combination that, in retrospect, we can recognize
as the weak field approximation of general relativity.

"It is also worth mentioning that Gerber's expression for the
gravitational potential predicts a substantially wrong (and
observationally excluded) deflection of light in a gravitational
field."

- Warren

Originally posted by Jaxom
So I withdraw that question, but ask this: if the redshift is due to time slowing, then there should never be a blueshift as light goes into a gravity well, correct? Light should redden whenever it is in heavy gravity, regardless of direction?
Yes, there's a blueshift when going deeper into the gravitational field.

- Warren

i wasn't refering to gerber for the correct explanation ... but since you do know how it can be done, I'll keep it at that

Jaxom – Let observers L and H be at fixed altitudes above a mass, with L lower than H. The observer L sees H’s light blueshifted, whereas H sees L’s light redshifted.

<i>i wasn't refering to gerber for the correct explanation...</i>

So, why should we care that some guy got it wrong before Einstein got it right? It changes nothing. Lots of people got it wrong.

On a related note, I respect a lot of the regular posters' knowledge and opinions. In trying to clarify my thoughts on the above topic, I ran across this site, and wondered about the validity of the science on it. To me, it looked valid, but I'm not an expert in physics either.

http://www.newtonphysics.on.ca/

On the topic, one of the papers discusses gravitational redshift as a product of the state of the atoms in the star, not by the light being changed as it climbs out of the well.

So, why should we care that some guy got it wrong before Einstein got it right?

except for the fact that he was a bit wrong, he had no physical explanation for what he did, something others now do have